Conventional practical resin dies for sheet metal pressing have been manufactured by a lamination method or a metal core method. First, the lamination method will be outlined below with reference to FIG. 14.
In FIG. 14, the numeral 20 denotes a core box. Within the core box 20 there is fixed a model 21 formed of wood, synthetic wood, gypsum, or the like. A releasing agent is applied to the inner surface of the core box 20 and the surface of the model 21, and a resin for the surface layer is applied onto the releasing agent to form a surface resin layer 22. When the surface layer resin was semi-cured, resin-impregnated fibers are stuck thereon to form a reinforcing layer 23. At this time, in conformity with local shapes of the die various resin-impregnated fibers are used properly so as to be compatible with such local shapes. For example, a woven cloth is used for a flat portion, or short fibers are used or short-cut woven cloth pieces are joined together for a curved portion. In a corner or a small curve, roving pieces cut in an appropriate length are embedded.
The space inside the reinforcing layer 23 is filled with a sand or short fibers-incorporated resin. Upon curing of that resin, there is obtained a sand core 24. Then, a levelling resin is applied onto the whole, which is then covered with a horizontal plate 25, allowing the resin to be cured. As a result, a levelling layer 26 is formed.
After curing of all the resins, the cured resinous body is taken out from the core box 20 to afford a finished resin die, in which the surface resin layer 22 is formed outside, the reinforcing layer 23 using a fiber-reinforced resin is formed inside the surface resin layer 22, the sand core 24 is formed inside the reinforcing layer 23, and the levelling layer 26 is formed as an upper surface layer.
The metal core method, which has also been used practically in addition to the lamination method, will now be outlined with reference to FIG. 15. A core 27 made by metal casting and having an external form smaller by 10 to 20 mm than the size of a die is prepared. In this case, an inlet 28 is formed in the core 27. A model 21 is fixed within the core box 20 and a releasing agent is applied to the model. Then, a small cured piece of a casting resin having a thickness corresponding to the thickness of the casting resin is disposed as a spacer, on which is then placed the core 27. The casting resin is poured from the inlet 28 to fill the space between the core 27 and the core box 20 and the space between the core 27 and the model 21. After the resin was cured and became integral with the core 27, the integral body is taken out to find that a resin layer 29 is formed on the outer periphery except the upper surface. By cutting and thereby levelling the upper surface there is obtained a resin die having desired shape and size.
In this resin die, the resin layer 29, which has a thickness of 10 to 20 mm, is formed on the outer periphery except the upper surface, and the metallic core 27 is present inside the resin layer 29. The resin layer 29 and the core 27 are bonded together as an integral body by virtue of a bonding force created at the time of curing of the resin
The sheet metal pressing work is based on a forming method wherein a blank holding pressure is applied to a blank such as a cold-rolled steel sheet inserted between the die surface and the blank holding surface, and in this state a punch is press-fitted in the blank to effect a plastic deformation of the blank. At the time of press-fitting of the punch, the blank undergoes a tensile deformation and a flexural deformation between the shoulder portion of the die and that of the punch, and is deformed plastically by those deforming forces. Consequently, large local loads are imposed on these shoulder portions in a forming operation. Therefore, a die for sheet metal pressing is required to have high load-resistant strength and abrasion resistance particularly at the die and punch shoulder portions. It is not too much to say that the durability of a die depends on the strength and abrasion resistance of its shoulder portion.
In the resin die produced by the foregoing lamination method, the reinforcing layer 23 for ensuring strength is formed inside the surface resin layer 22. In forming the reinforcing layer 23 there is used a woven cloth as a reinforcing fiber. However, the shape of the woven cloth incapable of expanding and contracting cannot be made coincident exactly with the curved shape of the die and punch shoulder portions which are curved three-dimensionally. Therefore, a woven cloth is not employable for the shoulder portions, and instead short fibers have been used, or small cut cloth pieces have been joined together in use. Or roving pieces cut in an appropriate length have been used in an embedded fashion.
In the case of using short fibers, it has heretofore been impossible to obtain a sufficient strength because the fibers to function as a reinforcing material are too short. Also in the case where cut cloth pieces are laminated while being joined together, it is impossible to expect a sufficient improvement of strength because of too short fibers; besides, a shear force is developed between laminated layers upon exertion of a load on the reinforcing layer, which may cause deterioration of the strength. Further, in the case of using roving pieces, it has heretofore been quite impossible to expect a transverse strength relative to the arranged direction of the fibers. In all of the above cases, because of a too low strength of the shoulder portion, the shoulder portion undergoes a plastic deformation and the wear thereof is amplified as the press work is repeated, and finally such resin dies became unemployable. Therefore, such conventional resin dies have been employable only in a small-volume forming operation such as trial manufacture.
On the other hand, in the case of a resin die manufactured by the metal core method, a resin which incorporates therein 20-25 vol % of a fine metallic powder is cast onto the surface of the metallic core 27 at a thickness of 10-20 mm to form the resin layer 29. The core 27 as a reinforcing member is strong because it is metallic. However, according to the structure of this resin die, the resin layer 29 is bonded to the outer periphery of the metallic core 27. Thus, it is a bonded body obtained by bonding different materials together, one being a metallic lump and the other a resinous lump. That is, both are greatly different in thermal expansion coefficient, heat storage property and rigidity, so with repetition of a load-acting cycle, the differences in physical properties and behaviors between the two materials gradually come up to the surface and finally cause delamination of the resin layer 29 at the shoulder portions. Further, since the shoulder portion is curved, part of the load acts as a component force in a direction of delaminating the resin layer 29. This also accelerates the delamination. Once the resin layer 29 is delaminated, the die will undergo a plastic deformation and the wear thereof will proceed acceleratively to an unemployable extent.
Accordingly, it is the object of the present invention to overcome the above-mentioned problems of the conventional resin dies and provide a resin die for sheet metal pressing having improved strength and durability of die and punch shoulder portions, as well as a method for producing such die.